1. Field of the Invention
This invention generally relates to piezoelectric thin-film resonators, filters having the aforementioned resonators and fabricating methods thereof, and more particularly, to a piezoelectric thin-film resonator, a filter having the aforementioned resonator, and a fabricating method thereof. Both the resonator and the filter are excellent in the mechanical strength, mountability, reliability, and productivity, and have an excellent orientation of the piezoelectric film.
2. Description of the Related Art
Wireless communication devices as represented by mobile telephones have been rapidly spreading, and accordingly a small-sized and lightweight resonator and a filter composed of a combination of the aforementioned resonators are increasingly demanded. Conventionally, a dielectric substance and a surface acoustic wave (SAW) filter have been mainly used. In recent years, however, a piezoelectric resonator and a filter composed of the piezoelectric resonators are expected to be used for the wireless communication device, because the piezoelectric resonator has excellent characteristics in high frequencies, and is capable of downsizing and forming a monolithic circuit.
FBAR (Film Bulk Acoustic Resonator) is known as one of the above-mentioned piezoelectric resonators. The FBAR has a main component of a laminated structure having an upper electrode film, a piezoelectric film, and a lower electrode film. The laminated structure is formed on a substrate. A gap (via hole or cavity) is formed in a region below the lower electrode, the region corresponding to where the lower electrode and the upper electrode face each other. This gap is formed by (wet or dry) etching a silicon substrate from the backside thereof. The silicon substrate is used as a device substrate. Alternatively, this gap is formed by wet etching a sacrifice layer provided on a surface of the silicon substrate.
If a high-frequency electric signal is applied between the upper electrode and the lower electrode, elastic waves are excited by the inverse piezoelectric effect, or are generated by the distortion caused resulting from the piezoelectric effect inside the piezoelectric film sandwiched between the upper electrode and the lower electrode. Then, the above-mentioned elastic waves are converted into the electric signals. The above-mentioned elastic waves are all  reflected on surfaces of the upper electrode (film) and the lower electrode (film) respectively in contact with air, generating in the longitudinal mode thickness excitation having a main displacement in a direction of thickness. In this device structure, the resonance occurs in the frequency in which a total thickness H of a thin-film structure is equal to an integral multiple (n times) of ½ wavelength of an elastic wave. The thin-film structure has the main component of the upper electrode film/the piezoelectric film/the lower electrode film provided above the gap. A propagation velocity V of the elastic wave varies depending on the substance, and a resonance frequency F is denoted by F=nV/2H. If the above-mentioned resonance phenomenon is utilized, the resonance frequency can be controlled by the film thickness as a parameter. It is possible to produce the resonator and the filter having desired frequency characteristics.
The upper and lower electrodes may employ metals such as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chrome (Cr), titanium (Ti) and the like, or may employ substances composed of a combination of the aforementioned metals. The piezoelectric film may employ aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3), or the like. Especially, it is preferable that aluminum nitride (AlN) or zinc oxide (ZnO) having a main axis of (002) orientation may be employed. Additionally, silicon, glass, and GaAs may be used for the device substrate.
As described, the piezoelectric thin-film resonator has to include the via hole or cavity located immediately below the lower electrode (or the dielectric film). Hereinafter, the via hole denotes a hole that pierces from the backside of the substrate through the surface of the substrate, and the cavity denotes a gap arranged in the vicinity of the surface of the substrate or arranged immediately below the lower electrode film (or the dielectric film). The conventional piezoelectric resonators are classified into a via hole type and a cavity type.
FIG. 1 is a cross-sectional view illustrating a schematic structure of the conventional piezoelectric thin-film resonator described in Electron. Lett., 1981, Vol. 17, p.p. 507-509 (hereinafter, referred to as Document 1). This structure employs a laminated structure. A lower electrode 13 of an Au—Cr film, a piezoelectric film 14 of a ZnO film, and an upper electrode 15 of an Al film are formed on a (100) plane of a silicon substrate 11 having thermally-oxidized films (SiO2) 12. A via hole 16 is arranged under the aforementioned laminated structure. The via hole 16 is formed by anisotropic etching from the backside of the (100) silicon substrate 11, with the use of KOH aqueous solution or EDP aqueous solution (compound liquid of ethylenediamine, pyrocatechol, and water).
It is to be noted that the via hole type of the piezoelectric thin-film resonator, as shown in FIG. 1, has the following problems. First, the anisotropic etching utilizes the characteristics that the etching rate of the (100) plane of the silicon substrate is rather faster than that of the (111) plane of the silicon substrate. Therefore, the anisotropic etching is an effective method only for etching the (100) plane of the silicon substrate. Second, the via hole inevitably has the sidewall shape having an angle of 54.7°, at which the (100) plane intersects with the (111) plane, and the device size becomes large. Also, the via hole degrades the mechanical strength, because a large part of the backside of the silicon substrate is etched to form the via hole. Third, if the filter is composed of multiple thin-film resonators arranged in proximity to each other, it is impossible to downsize the filter to a practically useful size, because it is difficult to downsize the resonators respectively. Fourth, the via hole provided on the silicon substrate makes it difficult to form other devices such as the inductance or capacitance on the same substrate, that is, the integration is not easy. Fifth, a special consideration is necessary for preventing the device having a low strength from damage in the dicing process for dicing the silicon substrate into the respective chips or in the mounting process for mounting on a package.
On the other hand, the cavity type of the piezoelectric thin-film resonator has the laminated structure on the sacrifice layer, the laminated structure having the upper electrode film, the piezoelectric film, and the lower electrode film, (and may further having the dielectric film as necessary) formed. The sacrifice layer is removed by etching, and the piezoelectric thin-film resonator having the cavity is thus formed.
FIG. 2 is a cross-sectional view illustrating a schematic structure of the cavity type of the aforementioned piezoelectric thin-film resonator (refer to Japanese Patent Application Publication No. 60-189307,  hereinafter referred to as Document 2). In this laminated structure, a lower electrode 23, a piezoelectric film 24, and an upper electrode 25 are formed on a substrate 21 having a thermally-oxidized film (SiO2) 22. A cavity 26 is formed below the laminated structure. The cavity 26 is formed by patterning a sacrifice layer of ZnO having an island shape in advance, forming the laminated structure on the sacrifice pattern, and removing the sacrifice layer provided below the laminated structure with acid.
Generally, the piezoelectric thin-film resonator that utilizes the longitudinal-mode thickness excitation as the FBAR has to include the piezoelectric film having an excellent orientation in order to obtain excellent resonance characteristics. The depth of the cavity normally requires several μm to several tens of μm, taking into consideration of the excitation displacement and the strain in the membrane portion. However, the surface is rough after the aforementioned thick sacrifice layer is formed, and the rough surface will degrade the orientation of the lower electrode 23 and that of the piezoelectric film 24 to be grown on or above the sacrifice layer. Moreover, a laminated body having the upper electrode film 25/the piezoelectric film 24/the lower electrode film 23 is provided on the SiO2 film 22 that serves as an underlying film, which protrudes upward and forms a bridge shape. This causes a problem in that the laminated body is weak against the mechanical vibration and has an inferior reliability in the practical use.
FIG. 3 is a cross-sectional view illustrating a schematic structure of a piezoelectric thin-film resonator disclosed in Japanese Patent Application Publication No. 2000-69594 (hereinafter referred to as Document 3). Document 3 has proposed the method for solving the problem on the orientation. In this laminated structure, a lower electrode 33, a piezoelectric film 34, and an upper electrode 35 are formed on a silicon substrate 31 having a thermally-oxidized film (SiO2) 32. A cavity 36 is formed below the laminated structure. The thin-film resonator of this structure is produced as follows.
A dent is first formed in a region on the surface of the silicon substrate 31. Then, the thermally-oxidized film (SiO2) 32 is formed on the surface of the silicon substrate 31 in order to prevent phosphorous included in PSG (phosphosilicate glass) used as the sacrifice layer from diffusing into the silicon substrate 31. After PSG of the sacrifice layer is deposited, the surface is polished and cleaned for mirror polish. Consequently, the lower electrode film 33, the piezoelectric film 34, and the upper electrode film 35 are sequentially stacked, and lastly PSG is removed. However, thus produced piezoelectric thin-film resonator has a high production cost. Besides, a troublesome polish process that has to remove slurry residue is included, and the number of fabricating processes is increased and the productivity is not excellent.